Onychomycosis in patients with diabetes mellitus: Etiology, clinical features, and treatment response

How to cite this article: Molina-Hernandez AL, Ramírez-Marín HA, Bonifaz A, Dominguez-Cherit JG. Onychomycosis in patients with diabetes mellitus: Etiology, clinical features, and treatment response. Our Dermatol Online. 2021;12(4):359-366. Submission: 08.05.2021; Acceptance: 28.08.2021 DOI: 10.7241/ourd.20214.3 Onychomycosis in patients with diabetes mellitus: Etiology, clinical features, and treatment response


INTRODUCTION
Onychomycosis affects 5.5% of the general population [1] and accounts for 30% of all superficial mycoses and 50% of all nail diseases [2,3]. The estimated prevalence is over 10% in the general population and about 40% in older adults [2]. The main causative agents of onychomycosis are dermatophytes; however, in those that do not respond to standard therapeutics, non-dermatophyte molds should be considered associated or causal agents [2].
One of the most studied predisposing factors is diabetes mellitus, with a frequency of onychomycosis of 31.5% in these patients [2,4]. It is now accepted that diabetes increases the relative risk of developing onychomycosis (from 1.5 to 2.8 times) compared to non-diabetic patients [2]. In a study on 400 patients in India, the prevalence of onychomycosis in diabetics was found to be 17%, being 6.8% in the control group [4]. Other factors related to an increased incidence of onychomycosis include old age, male sex, repeated nail traumas, genetic predisposition and underlying diseases such as immunodeficiency, and peripheral vascular disease [4]. Nail thickness has been positively associated with higher HbA1C levels [5], accelerating the typical subungual keratinization induced by fungal infection. Fungal infections exceed 50% of all types of infections among diabetic patients [6]. Diabetic patients with onychomycosis exhibit a higher percentage of gangrene and foot ulcers (12.2%) compared to those without onychomycosis (3.8%) [7,8].
Onychomycosis represents a risk for the development of the diabetic foot due its association with difficulty in nail clipping, causing the patient to injure themselves, producing an access route for bacterial agents [9]. Many of these patients show resistance to standard therapeutics (in some cases associated with incomplete previous treatments); they also have "polypharmacy," which represents a risk of pharmacological interactions in systemic management, or have contraindications for systemic management, hence they are candidates for topical treatment, after culture.
This study aimed to assess the clinical response to therapy and to evaluate with histopathology, direct examination with KOH and white-calcofluor, and culture the most frequent etiologic agents associated with the development of onychomycosis in patients with diabetes mellitus, both susceptible and resistant to standard treatment.

MATERIALS AND METHODS
A non-randomized, uncontrolled, open-ended, prospective cohort study was conducted on 46 patients with onychomycosis and diabetes mellitus treated at the Instituto Nacional de Ciencias Médicas y Nutrición "Salvador Zubirán," regardless of the time of evolution. Treatment was assigned according to clinical findings and specific indications for treatment, according to the type of onychomycosis and the number of affected nails (Table 1).
All patients were treated after their first dermatology evaluation with a follow-up at three months (in the case of systemic treatment) and at six months (in the case of topical treatment). Nails were photographed for follow-up and the samples were taken for direct examination with KOH 10% and calcofluor-white, and for mycological culture with agar for dermatophytes, SDA and potato dextrose agar. Furthermore, the distal edge of one of the affected nails was cut for histopathological study. Treatment adherence evaluation was conducted by interrogation during patient evaluations.

Ethics Statement
The authors confirm that the ethical policies of the journal, as noted on the journal's author guidelines page, were adhered to and the appropriate ethical review committee approval was received.

RESULTS
46 patients with diabetes and a clinical suspicion of onychomycosis were evaluated, 24 males (52.2%) and 22 females (47.8%), with an age ranging from 30 to 90 years, with a median of 57. Two patients were eliminated from the final analysis for death, and no sample was obtained for study from four other patients as they were lost on the last consultation.
In 40 patients evaluated at the end of the study, 2 showed no treatment adherence (5%), 9 patients (22.5%) showed 50% adherence, and 29 patients (72.5%) showed 100% adherence. It was considered 100% adherence in those who applied their treatment from the beginning, 50% in those who began the treatment late and 0% in those who failed to begin the treatment. Greater adherence was found in patients with systemic treatment (17 of the 29; 58.62%), compared to 13 patients with topical treatment (13 of the 29; 44.82).
Four patients received systemic treatment, with a minimum of one month of treatment (late onset) and a maximum of five months (prolonged treatment to clinical cure), with a median of 3; and among the 18 patients with topical treatment, the minimum treatment duration observed was two months and the maximum was six months, with a median of four; with a patient with indicated topical treatment which was not initiated. On clinical evaluation, we found no response in 8 patients (20%), a partial response in 14 patients (25%), and a complete response in 18 patients (45%) out  Considering the patients who completed their treatment in a consistent manner, completing the minimum treatment required for systemic (three months) or topical (six months) treatment and who, with or without clinical resolution, continued with positive diagnostic studies (direct examination, culture, or histopathological study), 4 (10%) were considered resistant to conventional treatment, without being able to consider the remaining 21 within this group, since they had not completed the indicated treatment by the late onset of the same.

DISCUSSION
Onychomycosis in diabetics is more than just a cosmetic problem and may be a limb-threatening infection if managed inappropriately [7]. A prospective study of 1285 diabetic patients found that the presence of onychomycosis is a significant predictor for the development of foot ulcers [8]. Onychomycosis in diabetic patients is more frequently observed in the presence of poor glycemic control and peripheral vascular disease [10]. A study found that the presence of onychomycosis in patients with diabetes is associated with subclinical atherosclerosis, which in diabetic patients represents the leading cause of death. [11]. The thickened, brittle nails typical of this infection are capable of causing damage to the surrounding skin that may go unnoticed due to coexistent neuropathy. Enlarged, dystrophic toenails may also put increased pressure on the underlying toe, compromising its tenuous vascular supply and causing pressure ulcers [11,12]. The pressure required to clip thickened toenails may cause inadvertent damage to the surrounding skin. If the patient is unable to perform appropriate foot hygiene, overgrowth of thickened toenails may cause damage to the surrounding skin as well. The subungual debris typical of distal subungual onychomycosis may become a reservoir for molds and bacteria [11,13], which may represent potential invaders of the compromised skin barrier. According to the Achilles Foot Screening Project, the most commonly clinically diagnosed foot diseases in the total population of patients who visited a dermatologist were fungal infections (35%), especially onychomycosis (23%) and tinea pedis (22%) [14]. Its etiology includes dermatophytes, yeasts, and fungi, in order of frequency. Regarding this, in a retrospective study lasting for 14 years on onychomycosis by molds, conducted at the Hospital General de Mexico by Bonifaz et al. [15], it was found that the most frequent clinical presentation was lateral and distal subungual in 69% of the cases, with Scopulariopsis brevicaulis in 34/78 patients and Aspergillus niger in 13/78 (16.6%) of the patients studied [15] Mixed infections and those caused by non-dermatophyte molds are more prevalent than previously thought, especially in warmer climates [16].
The results found in this study are consistent with the rest of the literature, since most of the patients presented total dystrophic onychomycosis, followed by distal subungual onychomycosis, and the most affected finger was the first of the left foot, with 41.3%.
Dermatophytic onychomycosis, predominates in male patients in a relation of 2:1. The global prevalence falls between 2-15% in adults and represents 50% of all ungual conditions [12].
Indications for systemic treatment include having three or more affected nail plates or having the condition on one nail at the level of the nail matrix [15]. The main difference in treatment is due to the fact that  some of the patients have chronic renal disease with creatinine clearance less than 50 ml/min, which contraindicates the use of terbinafine. Although it is the safest drug with the fewest drug interactions, there are no controlled studies in patients with creatinine clearance below 50 ml/min. For these, itraconazole may be used safely, although most of our population has polypharmacy, which sometimes makes management difficult because of drug interactions. It also requires greater vigilance with baseline and follow-up liver function tests.
Direct examination is performed after adding potassium hydroxide (KOH). KOH preparations have been reported to have a false-negativity rate of 5-15% [15]. Direct examination with KOH, although operator-dependent, correlates with culture results and with the number of positive histopathological studies, the latter being the gold standard in the diagnosis of onychomycosis, and further supports the positivity of cultures and rules out their contamination. Fungi may be better visualized by adding chlorazol black, periodic acid-Schiff (PAS), or calcofluor-white stain [17]. The use of the latter significantly increases the sensitivity and specificity of this technique for the detection of dermatophytes, yeasts, and non-dermatophyte fungi [18].
Among the main limitations of the study was the lack of confirmation through molecular biology studies, due to the lack of equipment in the institute. One of the key difficulties in interpreting this study is that the majority of fungi identified by culture are either not known to be nail pathogens or are highly rare pathogens. Despite diminution of immunity in some diabetics, the pathogens cited, such as Fusarium, cause onychomycosis in the face of profound neutropenia, which is rather different from that in diabetics. Another explanation might be that a dermatophyte in the nail has not grown due to the presence of contaminating fungi. Another is that the fungi found are secondary to some other cause of onychodystrophy.
The presence of bacterial clusters is interesting. It might be due to space under the nail, and the growth of bacteria such as S. aureus, which is key to the development of diabetic ulcers, is favored.
The increase in the number of histopathological findings in the control biopsies is thought to have been due to the fact that, initially, some of the patients were managed by "podiatrists," and, in addition to having been treated with bad nail cutting, they were given excessive filing of the nails, which made it difficult for some patients to take samples from their nails. When taking control samples, filing and cutting were intentionally suspended for a week prior to the evaluation for facilitation.
Diagnosis of onychomycosis is difficult because of the difficulty in isolating fungi from infected nails. The classical technique is performed with a curette. However, Shemer et al. showed in a study on 194 patients that, with a technique in which a channel 1-2 mm in size is perforated with an ungual drill, a medial, distal, and proximal sample can be taken, being effective for culture and therefore for diagnosis [19] (Fig. 2). Nail sampling is, in fact, a delicate procedure that requires the mycelium front to be reached by the instrument in order to provide positive cultures [20,21]. Histopathology of nail clippings stained with PAS also referred to as histomycology with PAS is the most sensitive method of diagnosing onychomycosis [22], while others point out that it is not more effective than direct examination [23,24]. Confocal laser-scanning microscopy is an emerging diagnostic technique. The aspect of dermatophytes appears as a network of lengthy structures with high reflection and the typical shape of hyphae [25,26]. Other interesting new tools in the diagnosis of onychomycosis are the dermatophyte test strip and Raman spectroscopy [22].
Immunosuppressed patients show variation in clinical presentation, as well as in causative agents. Additionally, fungi traditionally considered as non-pathogenic may be found as pathogens, often associated with high mortality, as in the case of Fusarium species that cause perionyxis and may provide a gateway for a disseminated infection [27]. As for the isolated agents in the cultures for this study, a large number of opportunistic fungi was found (Acremonium sp. Candida sp. (Fig. 3), Alternaria sp. Aureobasidium sp., Aspergillus sp., Candida guilliermondi, Candida albicans, Rhodotorula sp., Cryptococcus albidus  (Fig. 4), Fusarium sp. (Fig. 5), among others), which is probably associated with diabetes-associated immunosuppression (Table 4) [28,29].
In general, greater therapeutic failure was seen in onychomycosis caused by non-dermatophytes molds, followed by yeasts. Although the clinical improvement in these patients, both partial and total, was highly evident and larger than expected, the mycological and histological cure was far below from what was expected, hence we consider that a longer treatment time is required, either with the same medication or with topical and systemic combination therapy, and wait for a new culture and histological study. Approximately 20-25% of patients do not respond to the initial therapy and often switch to different treatments. An undetermined percentage of patients do not respond to any of the therapies [30]. The non-dermatophyte molds such as Fusarium spp., are the most resistant fungi to standard treatment for onychomycosis either topical or systemic [31], non-responders are defined as those patients who received treatment for six and three months without improvement in more than 50% of the nail unit [32,33].    It would be worth conducting a study on patients without diabetes mellitus to compare whether it is the immunosuppression of diabetes and its systemic affection as well as the foot abnormalities of the diabetic patient that cause more nail dystrophy, an increased fungal load, and treatment resistance.
Recently, a reproducible and objective system called the Onychomycosis Severity Index (OSI) was created to describe the extent and involvement of distal subungual onychomycosis, with points are given based on the area (%) of nail involvement, the proximity of the disease to the matrix, and the presence of dermatophytoma or subungual hyperkeratosis [16].
In conclusion, onychomycosis is a common ungual disease that affects a large part of the global population. Its prevalence is increasing, influenced in part by its association with older age, lifestyle, and concomitant diseases. It may cause disability and lead to mild to severe complications, among which the most important is the diabetic foot, which may condition osteomyelitis and sepsis, which justifies its early treatment. In addition, it requires a correct diagnosis, which includes clinical evaluation, direct examination, culture, and a histopathological study, since there are other non-fungal diseases of the nails that resemble onychomycosis and are over-treated, generating expensive treatments with potential adverse effects, which makes it mandatory to confirm the clinical diagnosis with direct examination, culture, and, if possible, histopathological study. The treatment of onychomycosis is one additional facet of diabetic care that promises to further improve outcomes in these patients.

ACKNOWLEDGMENTS
We would like to thank the ethics committee of the Instituto Nacional de Ciencias Médicas y Nutrición "Salvador Zubirán" for the approval of this study. We would also like to thank the microbiology department of the same institution for the interpretation of cultures and histopathological studies. All the photographs of patients with onychomycosis are published with their approval. The patients in this manuscript gave written informed consent to the publication of their case details. All authors revised the manuscript and provided critical feedbacks. All authors approved the final version of the manuscript for submission. All authors believe that the manuscript represent honest work.

Statement of Human and Animal Rights
All the procedures followed were in accordance with the ethical standards of the responsible committee on human experimentation (institutional and national) and with the 2008 revision of the Declaration of Helsinki of 1975.

Statement of Informed Consent
Informed consent for participation in this study was obtained from all patients.